if you were using XP before and are now using Vista that may explain part of the slowdown. Vista is about twice as system intensive as XP.
To speed things up, more RAM can't hurt and RAM is cheap. If you are using a 32 bit operating system, add 1 more GB of RAM; if you are using a 64 bit operating system, add 2 more GB of RAM. If you can add a second HDD, do that, and use the fastest HDD for the operating system, and the second HDD for everything else. There are several choices for fast HDD. I still like the 150GB Raptors. Faster still would be adding pairs of HDDs in a RAID configuration.
After that, read the sticky on overclocking. If your fastest stable OC is slow, consider a CPU cooler. Thermalright Ultra-120 Extreme currently has the best numbers. You will need a 120mm fan with that - get a Scythe S-Flex. They come in D, E and F flavors, you will want the F for a CPU cooler.
You probably don't need two fans for your CPU cooler, but I just saw another thread with two fans on an Ultra-120. Fans are cheap though, and if you decide to get two, install them on opposite sides of the cooler in a push pull configuration. You can buy the mounts for the second fan, or you can make your own out of paperclips following the pattern of the clips that are included for the first fan (I've read that properly bent paperclips do a better job holding the fan than the included clips).